In the past, the H configuration power amplifier has consisted of four power transistors connected in a symmetrical fashion about a motor, The H configuration circuit may be described as being broken up into two parallel branches. Each branch consisting of a first and second power transistor connected in a serial arrangement so that the emitter of the first transistor is connected to the collector of the second transistor. The collectors of the first transistors in each branch are connected to the same power source while the emitters of the second transistors in each branch are connected to ground. At the emitter to collector junction between the first and second transistors of each branch the motor is connected.
The motor is controlled by introducing signals to the bases of the four power transistors so that, for example, when the first transistors of one branch is made to conduct simultaneously with the second transistor of the other branch. This provides for the flow of current from the power source through the first transistor of one branch, through the motor, and through the second transistor of the other branch to ground. To reverse the direction of rotation of the motor, the two transistors conducting would be switched off and the other two transistors would be turned on.
A problem that arises in the amplifier is that during the operation of the amplifier, the first and second transistors in the same branch may be allowed to conduct simultaneously. If the two transistors in the same branch conduct simultaneously, the power will flow from the source to ground through this branch and bypass the motor. When the motor is bypassed, the motor load which aids in limiting the current will be eliminated. The elimination of the motor load can result in the amplifier being damaged or the power transistors being destroyed.
The shorting of the two transistors in the same branch may occur during a continual change of motor direction, during the switch off of the system, or in the event of current surges in the system.